These primary color components may be the additive primary color components red, green and blue, for example. Alternatively, these primary color components may be a luminance-only primary color and two chrominance-only primary colors, which by appropriate color matrixing can be converted to the additive primary colors. The invention can even have application to display processors operating with only one primary color component, such as luminance-only primary color.
In certain computers images are stored in image memory according to bit-map-organization. Each picture element or "pixel" is stored in a respective location in image memory. During each display field the image memory storage locations are addressed sequentially in synchronism with the tracking of scan lines on the computer display monitor; which conventionally uses a raster-scanned cathode ray tube or kinescope. In small computers the image memory is often included in the computer main memory, which is generally a dynamic memory. The display processor receives display information from an output port of the main memory. Recently so called video random access memories (VRAMs) have become commercially available. VRAMs are dual-ported memories having a random access input/output port and having a serial output port. This serial output port is at the end of a shift register in main memory, the successive stages of which shift register are side-loaded in parallel with descriptions of a scan line of successive image pixels during retrace intervals preceding line retrace intervals. The time taken for side-loading is essentially the same as the time for reading out from the random access port, but all the locations in a row are read out in parallel. Then, to supply pixel data to the display processor, this shift register is serially read out through the serial output port during each line trace interval. The shift register can be operated at high shift rate to supply pixel data at video rates, without the memory consuming excessive power. To get apparently higher shift rates, while keeping power consumption under control, the shift register can be constructed for banked operation using poly-phase shift clocks. Successive locations in the dual ported memory can be read row by row through the serial output port of the dual-ported memory at much higher rate, then, than the normal duty cycle of the memory operated for writing into or reading from a location via the random access port.
The other port of the main memory is the random access input/output port. This random access port is available for writing data into memory or reading data out of memory. Using this random access port, image data can be written into or deleted from the portions of computer main memory assigned to be the image memory. Also, this random access port is customarily used for access to computer main memory for computational tasks other than supporting the display. The cycle times for writing into and reading out from this random access port are much longer than one cycle of the pixel scan rate frequency, in dual-ported memories presently available.
Each of the pixel descriptions stored in image memory could comprise linear codings of the primary color components, but this usually involves long codes. A respective color map memory is provided for storing values of each of the primary color components. The image memory stores pixel descriptions which are "pointers" used as read addresses for the color map memories. A short read address code can access multi-bit linear codings of each of the primary color components to describe any color closely.
(One of the primary color components may be selected to be a luminance-only component. A map memory storing values of luminance-only component is sometimes called a luminance map memory, and only the map memories storing values of the other two primary color components are referred to as color map memories. In this specification the term "color map memory" will be used generically for both types of map memories.)
The color map memories are customarily operated as read-only memories during display. But it has been found convenient in the prior art to alter the contents of the color map memories to more closely fit the needs of a particular display. So, the color map memories are usually random access memories operated as read-only memories during display. These color map memories have in the prior art been re-written using data from the random access port of computer main memory. This places substantial constraints on the rate at which the color map memories may be rewritten. The complete re-writing of color map memories with many entries is convenient to do only during field retrace intervals in the display and is the custom in the prior art. Re-writing of color map memories with few entries has been done, but normally the field retrace interval has been too short to substantially re-write the color map memories.